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[Stack Switching] RemoveUnusedModuleElements: Fix closed world analysis of continuation functions (WebAssembly#7841)
cont.new is a way to execute code, so we need to be aware of that, otherwise we might think a function will never be called, and turn its body unreachable.
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‎scripts/test/fuzzing.py‎

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@@ -119,6 +119,7 @@
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'cont_export.wast',
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'cont_export_throw.wast',
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'type-merging-cont.wast',
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'remove-unused-module-elements-cont.wast',
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# TODO: fix split_wast() on tricky escaping situations like a string ending
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# in \\" (the " is not escaped - there is an escaped \ before it)
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'string-lifting-section.wast',

‎src/passes/RemoveUnusedModuleElements.cpp‎

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@@ -190,6 +190,16 @@ struct Noter : public PostWalker<Noter, UnifiedExpressionVisitor<Noter>> {
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auto type = curr->ref->type.getHeapType();
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noteStructField(StructField{type, curr->index});
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}
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void visitContNew(ContNew* curr) {
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// The function reference that is passed in here will be called, just as if
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// we were a call_ref, except at a potentially later time.
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if (!curr->func->type.isRef()) {
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return;
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}
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noteCallRef(curr->func->type.getHeapType());
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}
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};
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// Analyze a module to find what things are referenced and what things are used.

‎test/lit/passes/remove-unused-module-elements-cont.wast‎

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;; NOTE: Assertions have been generated by update_lit_checks.py --all-items and should not be edited.
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;; RUN: foreach %s %t wasm-opt --remove-unused-module-elements --closed-world -all -S -o - | filecheck %s
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;; RUN: foreach %s %t wasm-opt --remove-unused-module-elements -all -S -o - | filecheck %s --check-prefix OPEN_WORLD
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;; Test both open world (default) and closed world. In a closed world we can do
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;; more with function refs, as we assume nothing calls them on the outside, so
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;; if no calls exist to the right type, the function is not reached.
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(module
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;; CHECK: (type $func (func))
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;; OPEN_WORLD: (type $func (func))
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(type $func (func))
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;; CHECK: (type $cont (cont $func))
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;; OPEN_WORLD: (type $cont (cont $func))
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(type $cont (cont $func))
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;; CHECK: (elem declare func $func)
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;; CHECK: (tag $tag (type $func))
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;; OPEN_WORLD: (elem declare func $func)
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;; OPEN_WORLD: (tag $tag (type $func))
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(tag $tag (type $func))
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;; CHECK: (export "run" (func $run))
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;; CHECK: (func $func (type $func)
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;; CHECK-NEXT: (nop)
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;; CHECK-NEXT: )
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;; OPEN_WORLD: (export "run" (func $run))
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;; OPEN_WORLD: (func $func (type $func)
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;; OPEN_WORLD-NEXT: (nop)
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;; OPEN_WORLD-NEXT: )
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(func $func
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;; This function is only ever referred to by a ref.func that is passed into
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;; a cont.new. We should not modify this to unreachable, which we would do if
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;; we didn't realize it will execute (in closed world).
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(nop)
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)
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;; CHECK: (func $run (type $func)
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;; CHECK-NEXT: (drop
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;; CHECK-NEXT: (block $block (result (ref $cont))
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;; CHECK-NEXT: (resume $cont (on $tag $block)
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;; CHECK-NEXT: (cont.new $cont
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;; CHECK-NEXT: (ref.func $func)
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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;; CHECK-NEXT: (return)
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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;; OPEN_WORLD: (func $run (type $func)
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;; OPEN_WORLD-NEXT: (drop
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;; OPEN_WORLD-NEXT: (block $block (result (ref $cont))
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;; OPEN_WORLD-NEXT: (resume $cont (on $tag $block)
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;; OPEN_WORLD-NEXT: (cont.new $cont
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;; OPEN_WORLD-NEXT: (ref.func $func)
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;; OPEN_WORLD-NEXT: )
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;; OPEN_WORLD-NEXT: )
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;; OPEN_WORLD-NEXT: (return)
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;; OPEN_WORLD-NEXT: )
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;; OPEN_WORLD-NEXT: )
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;; OPEN_WORLD-NEXT: )
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(func $run (export "run")
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;; No changes are expected here.
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(drop
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(block $block (result (ref $cont))
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(resume $cont (on $tag $block)
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(cont.new $cont
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(ref.func $func)
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)
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)
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(return)
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)
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)
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)
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)
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(module
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;; CHECK: (type $func (func))
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;; OPEN_WORLD: (type $func (func))
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(type $func (func))
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;; CHECK: (type $cont (cont $func))
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;; OPEN_WORLD: (type $cont (cont $func))
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(type $cont (cont $func))
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;; CHECK: (tag $tag (type $func))
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;; OPEN_WORLD: (tag $tag (type $func))
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(tag $tag (type $func))
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;; CHECK: (export "run" (func $run))
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;; CHECK: (func $run (type $func)
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;; CHECK-NEXT: (drop
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;; CHECK-NEXT: (block $block (result (ref $cont))
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;; CHECK-NEXT: (resume $cont (on $tag $block)
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;; CHECK-NEXT: (cont.new $cont
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;; CHECK-NEXT: (ref.null nofunc)
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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;; CHECK-NEXT: (return)
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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;; CHECK-NEXT: )
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;; OPEN_WORLD: (export "run" (func $run))
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;; OPEN_WORLD: (func $run (type $func)
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;; OPEN_WORLD-NEXT: (drop
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;; OPEN_WORLD-NEXT: (block $block (result (ref $cont))
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;; OPEN_WORLD-NEXT: (resume $cont (on $tag $block)
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;; OPEN_WORLD-NEXT: (cont.new $cont
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;; OPEN_WORLD-NEXT: (ref.null nofunc)
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;; OPEN_WORLD-NEXT: )
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;; OPEN_WORLD-NEXT: )
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;; OPEN_WORLD-NEXT: (return)
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;; OPEN_WORLD-NEXT: )
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;; OPEN_WORLD-NEXT: )
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;; OPEN_WORLD-NEXT: )
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(func $run (export "run")
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;; We should not error on a null function reference in the cont.new.
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(drop
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(block $block (result (ref $cont))
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(resume $cont (on $tag $block)
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(cont.new $cont
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(ref.null $func)
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)
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)
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(return)
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)
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)
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)
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)
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